1. Field of the Invention
The present invention relates to a reactor having a structure wherein heat exchange tubes for flowing a heat medium for cooling or heating are provided in a reactor vessel and the heat exchange tubes are connected to a tubesheet, and the reactor vessel contains a corrosive reaction fluid in the inside thereof. Especially, the present invention relates to a reactor for urea condensing and synthesis which contains a mixed gas including ammonia and carbon dioxide.
2. Background of the Invention
Well-known methods of producing urea include reacting ammonia and carbon dioxide in a synthesis reactor at a urea synthesis pressure and urea synthesis temperature, separating unreacted ammonium carbamate (an intermediate product in the urea synthesis) as a gas mixture of ammonia and carbon dioxide from the obtained urea synthesis mixture, condensing and recycling the gas mixture to the urea synthesis zone, and obtaining a urea solution from the urea synthesis mixture from which the ammonium carbamate and the like have been removed. Various proposals have been made recently regarding such methods in order to realize a more economical urea plant. Such proposals include reducing the number of equipment in a high-pressure synthesis loop of a urea plant, and the total equipment volume, the installation area, and the height of the plant by developing a condensation and synthesis reactor in which an apparatus for condensing the gas mixture, which includes ammonia and carbon dioxide gas, and an apparatus for urea synthesis are integrated.
Patent Document 1 (Japanese Patent Laid-Open No. 2002-20360) and the corresponding Patent Document 2 (U.S. Pat. No. 6,476,262) describe combining a condenser and a synthesis reactor, which were conventionally provided as separate pieces of equipment, by arranging a tube bundle for cooling over the middle portion to bottom portion of a vertical synthesis reactor. A gas mixture including unreacted ammonia and carbon dioxide gas and an absorbing medium are introduced from the bottom portion of the reactor, and feedstock ammonia is supplied to the bottom and the middle portions of the reactor. The reaction heat of ammonium carbamate formation (ammonium carbamate is an intermediate product) is removed by cooling by way of the tube bundle, thereby promoting condensation of the gas mixture, whereby the synthesis reaction is made to further progress over the middle portion to top portion of the synthesis reactor.
Patent Document 3 (U.S. Pat. No. 5,767,313) describes combining the condenser and the synthesis reactor, which were conventionally provided as separate pieces of equipment, by arranging a tube bundle for cooling on one side of a horizontal condensation and synthesis reactor. Feedstock ammonia is introduced from the side on which the tube bundle is provided so that the feedstock ammonia successively flows through many baffles to the side opposite the tube bundle, a gas mixture is flowed including unreacted ammonia and carbon dioxide gas from the bottom of the entire reactor so that the gas mixture mainly condensates at the tube bundle, and further carrying out the synthesis reaction in the remaining portion of the reactor.
Patent Document 4 (European Patent No. 0155735) describes an apparatus in which the condenser and the synthesis reactor are not completely combined. But this apparatus is the same as described in Patent Document 3 in that a synthesis reaction is also started at the condenser in addition to condensation of the gas mixture.
In general, a urea synthesis mixture containing ammonium carbamate, which is an intermediate product in urea synthesis, is extremely corrosive to metals. For that reason, for the equipment used in urea synthesis loop (e.g., the synthesis reactor, condenser and stripper), the parts that come into contact with the urea synthesis solution are all made of a corrosion-resistant metal, such as high-chromium austenite steel, a dual-phase alloy, titanium or 316L austenite steel, which are corrosion-resistant to the urea synthesis mixture. On the other hand, because the required pressure for urea synthesis is high at about 12.5 MPaG to 35 MPaG (the “G” used in units of pressure refers to “gauge pressure”), the basic material for the pressure-resistant portions of the equipment is preferably carbon steel or low-alloy steel, which are economical and reliable. For this reason, methods have been employed until now wherein carbon steel or low-alloy steel is used for the pressure-resistant portions of the equipment in a urea synthesis loop and the inner surface of all the portions in contact with the urea synthesis mixture is lined with a corrosion-resistant metal.
Even in recently developed urea condensation and synthesis reactors, the material composition of the equipment body is basically the same as above. However, the urea condensation and synthesis reactor differs from the conventional art in that the tube bundle for cooling is inserted into a highly corrosive urea synthesis mixture, whereby the portion where the tubes are fixed to the tubesheet requires special consideration. That is, the tubes made of corrosion-resistant metal must be fixed by a full-penetration weld with no crevice to the corrosion-resistant metal side of the tubesheet (i.e. the side in contact with the urea synthesis mixture) which is formed of two materials, namely a corrosion-resistant metal and carbon steel or low-alloy steel. In practice, an inner bore weld in which the welding torch is inserted from the tube side (channel side) of the tubesheet may be employed as a practical welding method. This is because of the following two reasons.
In order to withstand the high pressure of urea synthesis pressure and resist corrosion from the urea synthesis mixture, the tubesheet for attaching the tube bundle is a plate in which carbon steel or low-alloy steel serves as a material for the pressure-resistant portion and a corrosion-resistant metal is put onto the interior portions of the equipment in contact with the urea synthesis mixture. Usually, deposit (overlay) welding is carried out using a corrosion-resistant metal onto the carbon steel or low-alloy steel of the pressure-resistant portions to fabricate a tubesheet in which two types of material have been combined. To attach a tube made of a corrosion-resistant metal to this tubesheet, a usual attaching method using an tube expanding or welding from the tube side (channel side) cannot be used, and it is necessary to attach the tube by directly welding to the corrosion-resistant metal layer of the tubesheet. This is because in usual methods in which the tube is attached to the tubesheet using an tube expanding or welding from the tube side, the corrosive urea synthesis mixture penetrates into the carbon steel or low-alloy steel portion, which serves as the pressure-resistant portion of the tubesheet, whereby the important pressure-resistant portion cannot be protected from corrosion.
High-chromium austenite steel (25Cr-22Ni-2Mo steel and the like), dual-phase alloys (25Cr-7Ni-3Mo steel and the like), titanium and 316L austenite steel are economical and often used as a corrosion-resistant material against urea synthesis mixtures. These materials form a passive film on the metal surface using dissolved oxygen in the urea synthesis mixture, thereby achieving corrosion resistance against the urea synthesis mixture. Therefore, if a narrow crevice exists on the surface of the metal, dissolved oxygen is not supplied to inside the crevice, whereby a passive film is not formed at that portion. This results in crevice-corrosion (i.e. only the crevice portion is selectively corroded). For this reason, the weld for welding the above-described tube to the corrosion-resistant metal layer of the tubesheet must employ a welding method which leaves no crevices on the urea synthesis mixture side.